Troubleshooting of Cell Cycle Staining Flow Cytometry

Detailed troubleshooting tips to help you resolve flow cytometry issues for cell cycle analysis purpose. Though the troubleshooting tips provided here cover many different problems you may encounter, we do not expect it to be the exclusive solution to any problems. We hope that you will find the information beneficial to you and useful as a reference guide in troubleshooting cell cycle staining flow cytometry problems. If you need more assistance with your experiments, please contact us for additional help.

Click for the Protocol of Cell Cycle Staining Flow Cytometry.

Contents

Problem 1: No Signal / Weak Fluorescence Intensity
Problem 2: High Fluorescence Intensity
Problem 3: High Background
Problem 4: Non-Specific Staining
Problem 5: Low Event Rate
Problem 6: High Event Rate
Problem 7: Unusual Scatter Profiles
Problem 8: Loss of Epitope
Problem 9: Two or More Cell Populations Are observed When There Should Be Only One Population
Problem 10: When running samples to examine cell cycle distribution the histogram for DNA content does not resolve the distinct phases of the cell cycle, i.e., G0/G1, S and G2/M phases.
Other Possible Problems
Tips

No Signal / Weak Fluorescence Intensity

Cause	Solution
Insufficient detection antibody	Increase the concentration of detection antibody.
Sub-optimal antigen-antibody binding	Check the species specificity of the antibody. Optimize the antibody incubation time and temperature. Consider using biotinylated primary antibodies and an additional biotin-streptavidin step to amplify the signal.
The surface antigen is getting internalized	Perform all steps at 4°C and use ice cold reagents.
Target protein not present or expressed at low level	Ensure tissue or cell type expresses target protein and that it is present high enough to be detected.
Antibody storage	Ensure that all antibodies have been stored correctly. Ensure that commercial antibodies have not exceeded their date of expiration.
Higher cell concentration	Adjust cell population to recommended density.
Inadequate fixation and/or permeabilization of sample.	70% ethanol (ice-cold) can sometimes be used as an alternative method of fixation, particularly when performing cell cycle analysis.
Others	If antigen expression is weak, select an antibody that is conjugated to a brighter fluorochrome. If the fluorochrome used is phycoerythrin or allophycocyanin-based, ensure that the product has not been frozen. Ensure that correct laser is being used to excite fluorochrome, and that correct channel is being used to analyze emissions.

High Fluorescence Intensity

Cause	Solution
High antibody concentration	Reduce the amount of antibody added to each sample. Too much antibody will result in high non-specific binding. Use appropriate positive and negative controls.
Inadequate blocking	Dilute antibodies in blocking solution.
A high expressing antigen is paired with bright fluorochrome	Always pair strong antigens with dimmer fluorochromes such as FITC or Pacific Blue.
The fluorescent signal is under-compensated	Use MFI alignment instead of visual comparison to compensate.

High Background

Cause	Solution
The gain set is too high, the offset is too low	Use the positive control to set up the flow cytometer correctly again. Use the offset to reduce background from small particles and reduce the gain to decrease the signal but do this within reason.
Excess antibody	Decrease antibody concentration. Add a detergent to the wash buffers to ensure washing away of excess antibody.

Non-Specific Staining

Cause	Solution
High autofluorescence	Always include an unstained control to subtract the auto-fluorescence signal. For cells with naturally high auto-fluorescence, use fluorochromes that emit in the red channel where auto-fluorescence is minimal. Use very bright fluorochromes for these cell types to amplify the signal above the auto-fluorescence level. Avoid storing the cells in a fixative solution for long durations and analyze cells soon after staining if possible.
Non-specific cells are targeted	Include an isotype control to subtract any Fc binding signal. Block the FcR on cells with Fc blockers, BSA or FBS prior to antibody incubation. Additional washing steps.
Excess, unbound antibodies are present in the sample	Wash cells adequately after every antibody incubation step.
Presence of dead cells	Always sieve the cells once before acquiring and sorting to remove any dead cell debris. Include viability dyes like PI or 7-AAD to gate out any dead cells. Use freshly isolated cells as opposed to frozen cells whenever possible.

Low Event Rate

Cause	Solution
Low number of cells	Optimal cell concentration for each sample should be around 1x10⁶ cells/mL.
Cells clumped/blocking tubing	Pipet the samples gently before staining and again before running the cytometer.

High Event Rate

Cause	Solution
High number of cells	Optimal cell concentration for each sample should be around 1x10⁶ cells/mL.
Contamination	Repeat the staining procedures.

Unusual Scatter Profiles

Cause	Solution
The cells are lysed or damaged	Optimize sample preparation to avoid cell lysis. Do not vortex or centrifuge cells at high speeds. Use fresh buffers. Avoid storing the stained cells for long durations.
Bacterial contamination	Store stained cells and antibodies properly to avoid bacterial growth. Bacteria will exhibit low levels of auto fluorescence. Practice proper sterile cell culture techniques to prevent bacterial contamination.
Presence of dead cells	Always sieve the cells once before acquiring and sorting to remove any dead cell debris. Use freshly isolated cells as opposed to frozen cells whenever possible.
Incorrect instrument settings for scatter	Ensure proper instrument settings are loaded prior to acquisition. Use fresh, healthy cells to correctly set the FSC and SSC settings for that cell type.

Loss of Epitope

Cause	Solution
Sample was not kept on ice	Keep antibodies at 4°C to prevent loss of activity. This also prevents active phosphatases and proteases from altering the epitope of interest.

Two or More Cell Populations Are observed When There Should Be Only One Population

Cause	Solution
Cell doublets are present in the sample	Mix the cells gently before the staining process and again before running them on the cytometer using a pipette. Sieve or filter cells to remove clumps.
There is more than one cell population present which is expressing the target protein	Check the expected expression levels from the cell types that are present in the sample and ensure adequate cell separation if necessary.

When running samples to examine cell cycle distribution the histogram for DNA content does not resolve the distinct phases of the cell cycle, i.e., G0/G1, S and G2/M phases.

Cause	Solution
Incorrect flow rate	Ensure that your samples are being run at the lowest flow rate setting on your cytometer. High flow rates will give rise to high coefficients of variation (CVs), leading to a loss of resolution of the different phases of the cell cycle.
Insufficient staining with Propidium Iodide/RNase (PI) solution.	Resuspend cell pellet directly in PI/RNase solution and incubate for at least 10 min. If PI cannot be used due to fluorescent channel configurations, change an alternate DNA dye.
Cells are not proliferating.	Cells should be harvested during asynchronous and exponential growth to ensure that all phases of the cell cycle are accurately represented.

Other Possible Problems

Cause	Solution
Population shifts during measurement	Too much PI in your stained sample. Decrease the amount of PI.
Population shifts between samples	Cell numbers are different. Optimal cell concentration for each sample should be around 1x10⁶ cells/mL.
Large CVs and weak resolution of histograms	Too much free DNA/ RNA in the cell suspension.
Large CVs	The flow rate is to high. Optimize the flow rate.

Tips

Cells should be kept as concentrated as possible to allow the lowest sample pressure differential to be used. This will ensure that the core sample stream is as narrow as possible and give optimal CVs.
70% ethanol should not be made with PBS as this causes protein precipitation on fixation. Use 70 parts absolute ethanol to 30 parts distilled water.
Single parameter DNA analysis will not yield any kinetic information, nor will it be able to distinguish between cells in very early or late S phase from cells in G1 and G2 phase respectively. Nor can you distinguish between G2 and Mitotic phase cells. For this information, a bromodeoxyuridine (BrdU) technique should be used. Alternatively, you can combine DNA analysis with a cell cycle phase-specific marker.